BIVALVIA. 151 
CarpiuM,* Linnaeus. 
PrceruncuLus. Adanson, 1757. 
Crrastes and CrrastopEerMA. Poli, 1795. 
Isocarpta, (sp.) Klein, 1753. 
Carpissa. Megerle, 1811. 
Apuropita. Lea, 1833. 
LunuLacarpium. Miinst., 1840. 
La&vicarpium. Swains, 1840. 
HeMIcarDIUM. Id. 
ACARDO. id: 
PAPYRIDEA. Id. 
Serriees. Beck, sec. Gould. 
Monopacna (part) and Dipacna. ichw., 1841. 
Generic Character. Shell equivalve, sub-equilateral, more or less heart-shaped ; 
generally closed, sometimes gaping posteriorly: usually inflated, orbicular, or ovate, 
costated, costz variable, from nearly obsolete to large and prominent, sharp, round and 
naked, often ornamented with scales or tubercles; margin dentated or crenulated ; 
hinge composed of two cardinal teeth in each valve, and two remote and prominent 
lateral teeth. Ligament external. Impression of the mantle without a sinus. 
Animal of the general form of the shell, its mantle open in front, with the margins 
generally plain, occasionally fringed, particularly towards the posterior, and around 
the syphons; these are short, and slightly separated, with the margins of one or both 
always fringed ; foot large, sub-cylindrical, and bent at nearly a right angle, possessing 
the form of an inflected arm or elbow-joint. 
Kichwald has described some species from the Caspian sea, which he has separated 
into three genera, under the names of Didacna, Monodacna, and Adacaa, depending for 
his characters upon the numerical presence, or the absence of the hinge-denticles. The 
animals of the latter division appear deserving of separation, being furnished with 
elongated syphons, and consequent indenture of the mantle mark. Some fossils 
also from the Paleozoic rocks, justly erected into a Genus by Professor Phillips under 
the name Plewrorhynchus, much resembled the general character of cockles, but were 
probably furnished with elongated syphons, as the shell is greatly produced on the 
posterior side. Species possessing undoubted characters of this Genus, have been 
obtained from the Middle Secondary Formations, and they are largely developed 
in the Tertiaries, while from the present seas not less than 200 species have been 
obtained. Their range geographically takes in nearly the whole surface of the Globe, 
and they are met with in Estuaries as high as low-water mark, while some are 
inhabitants of the sea at depths not exceeded by any other Molluscs. They generally 
frequent sand or sandy-mud bottom, where they often congregate in prodigious numbers. 
* Etym. xapéca, the heart. 
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